In order to construct a rotating support section for a variety of machinery, a back-to-back double-row angular ball bearing 1 such as shown in FIG. 7 is widely used. This double-row angular ball bearing 1 has; an outer ring 3 having double-row outer ring raceways 2 on the inner peripheral surface, a pair of inner rings 5 each having an inner ring raceway 4 on the outer peripheral surface, a plurality of balls 6 provided so as to freely roll between the double-row outer ring raceways 2 and the inner ring raceways 4 of the pair of inner rings 5, and a pair of cages 7 for holding the balls 6. In such a double-row angular ball bearing 1, for example, the outer ring 3 is fitted into a housing 8, and the pair of inner rings 5 are fitted onto a rotating shaft 9. The rotating shaft 9 is rotatably supported inside the housing 8.
The outer ring 3 and the pair of inner rings 5 which constitute such a double-row angular ball bearing 1 are processed to a predetermined shape and size by, for example, performing a forging process, a rolling process, machining process and grinding process in the well-known manner described in JPH09-176740, JPH09-280255, JPH11-140543, JP2002-079347, JP2003-230927 and the like. For example, the outer ring 3 is conventionally manufactured by the processes shown in FIG. 8. First, this conventional manufacturing method for a bearing outer ring is described.
In the conventionally known manufacturing method of a bearing outer ring shown in FIG. 8 (A) to FIG. 8 (F), first a columnar raw material 10 as shown in FIG. 8 (A) is obtained by cutting a long piece of a raw material into predetermined lengths. The size of the raw material 10 is determined according to the type of bearing outer ring, however, normally, the ratio of the diameter to the axial length is approximately 5:4 to 5:6.
Next, the raw material 10 is subjected to an upsetting process by compressing the raw material 10 in the axial direction between opposing pressing surfaces of a pair of dies, to obtain a first intermediate material 11 whose outer peripheral surface is a convex circular arc as shown by FIG. 8 (B). Normally, the first intermediate material 11 is such that the axial length is compressed to 70 percent or less of the axial length of the raw material 10.
Next, this first intermediate material 11 is subjected to a backward extrusion process shown by FIG. 8 (C) to FIG. 8 (D) to obtain a second intermediate material 12 shown by FIG. 8 (D).
The backward extrusion process is performed by compressing the center section in the radial direction of the first intermediate material 11 in the axial direction, between a die 13 and a punch 14, and plastically deforming the section closer to the outside in the radial direction in a direction opposite to the pushing direction of the punch 14. The die 13 has a bottomed cylindrical shape, and is provided with a circular base plate portion 15 and a peripheral wall portion 16 which extends upward from the outer peripheral edge section of the base plate portion 15. An annular groove 17 is formed around the entire periphery of the section closer to the outside in the radial direction of the base plate portion 15. The inner peripheral surface of the peripheral wall portion 16 has a stepped shape in which an inner periphery large diameter portion 18 on the opening side (from the center section to the top end section) is connected to an inner periphery small diameter portion 19 on the base plate 15 side (in the bottom end section) through an inner periphery inclined portion 20 in the center section in the axial direction. The inner periphery small diameter portion 19 is positioned upon the same cylindrical surface as the inner peripheral surface on the outer-diameter side of the annular groove 17. The outer peripheral surface of the punch 14 also has a stepped shape in which an outer periphery small diameter portion 21 in the section closer to the distal end (the lower half section) is connected to an outer periphery large diameter portion 22 in the section closer to the base end (the upper half section) through an outer periphery inclined portion 23 in the center section in the axial direction. The die 13 and the punch 14 having these respective constructions are concentrically secured to and supported by a table and ram of a pressing machine. In other words, the die 13 is secured to the top surface of the table and the punch 14 is secured to the bottom end face of the ram.
When performing this backward extrusion process, with the punch 14 and the ram in an elevated state, the first intermediate material 11 is set inside the die 13. In the case of the conventional manufacturing method, the outer diameter of the first intermediate material 11 is smaller than the inner diameter of the inner periphery small diameter portion 19, at least part of the section closer to the bottom end which enters inside the inner periphery small diameter portion 19. Accordingly, in a state with the first intermediate material 11 set inside the die 13, the bottom surface of the first intermediate material 11 comes into contact with the radially inside section with respect to the annular groove 17 of the top surface of the base plate 15, as shown by FIG. 8 (C). Then from this state, the punch 14 is lowered by the ram, thereby compressing the center portion of the first intermediate material 11 in the axial direction between the distal end surface of the punch 14 and the top surface of the base plate 15 of the die 13, as shown by FIG. 8 (D).
The metal material which is extruded radially outward from the space between the top surface of the base plate 15 and the distal end surface of the punch 14 by this compressing action, moves in the opposite direction (upward) to the pushing direction of the punch 14, together with the metal material present in the section closer to the outside in the radial direction of the first intermediate material 11. Thus, the metal material which moves in the opposite direction to the pushing direction of the punch 14 follows the shape of the outer peripheral surface of the punch 14 and the inner peripheral surface of the peripheral wall portion 16 to form a stepped cylinder of which the inner and outer peripheral surfaces are constructed by stepped cylindrical surfaces. Furthermore, part of the metal material enters inside the annular groove 17, so that the shape of this portion becomes a rim shape. By the backward extrusion process performed in this manner, a second intermediate material 12 is obtained with the overall shape of a bottomed cylinder in which the inner and outer peripheral surfaces are constructed by stepped cylindrical surfaces as shown by FIG. 8 (D). The thickness in the axial direction of the base portion 24 of the second intermediate material 12 is approximately 10% to 20% of the axial length of the first intermediate material 11. Moreover, the thickness in the radial direction of the cylindrical portion of the second intermediate material 12 is approximately 15% to 25% of the diameter of the first intermediate material 11.
Next, this second intermediate material 12 is subjected to a punching process which punches out a base portion 24 of the second intermediate material 12, to produce a third intermediate material 25 with the shape of a stepped cylinder as shown by FIG. 8 (E). This punching process is performed by using a pressing machine to drive a blanking punch through the second intermediate material 12.
After the third intermediate material 25 is produced in this manner, the third intermediate material 25 is subjected to a cold rolling forming (CRF) to produce a fourth intermediate material 26 as shown by FIG. 8 (F). In this cold rolling forming, for example, the third intermediate material 25 is fitted inside an outer diameter side roller which has an inner diameter matching the outer diameter (on the large diameter side) of the third intermediate material 25 and whose inner peripheral surface is constructed by a cylindrical surface. Moreover an inner diameter side roller which has an outer diameter sufficiently smaller than the inner diameter of the third intermediate material 25 and whose outer peripheral surface generating line shape corresponds with the generating line shape of the inner peripheral surface of the fourth intermediate material 26 (with the concave-convex shapes thereof being reversed to each other) is pushed against the inner peripheral surface of the third intermediate material 25. Then, the inner diameter side roller is pushed while rotating against the inner peripheral surface of the third intermediate material 25. Because the outer diameter side roller is supported in a manner which allows only rotation (with the displacement in the radial direction prevented), then with rotation of the inner diameter side roller, the third intermediate material 25 rotates together with the outer diameter side roller. As a result, the generating line shape of the outer peripheral surface of the inner diameter side roller is transferred to the entire periphery of the inner peripheral surface of the third intermediate material 25, and the outer peripheral surface of the third intermediate material 25 is processed into a cylindrical surface.
This rolling process may also be performed by sandwiching part in the circumferential direction of the third intermediate material 25 between a pair of rollers rotating in mutually opposite directions, and applying pressure to the rollers to push them towards each other so as to transfer the shape of the outer peripheral surfaces of the rollers to the inner and outer peripheral surfaces of the third intermediate material 25. In either case, the fourth intermediate material 26 as shown by FIG. 8 (F) is obtained. In this fourth intermediate material 26, the outer peripheral surface is constructed by a cylindrical surface whose outer diameter does not vary substantially in the axial direction, and the inner peripheral surface has an inclined shape in which the inner diameter of the center section in the axial direction is the smallest and the inner diameter increases gradually towards both end sections in the axial direction. The thickness in the radial direction of the both end sections in the axial direction of the fourth intermediate material 26 is approximately 70 percent to 80 percent of the thickness in the radial direction of the cylindrical portion of the second intermediate material 12, and the thickness in the radial direction of a portion having the smallest inner diameter of the center section in the axial direction of the fourth intermediate material 26 is approximately 90 percent to 120 percent of the thickness in the radial direction of the cylindrical portion of the second intermediate material 12.
The thus obtained fourth intermediate material 26 is subjected to the required finishing processes to thereby complete the outer ring 3 which constitutes the double-row angular ball bearing 1, as shown in FIG. 7. That is to say, by shaving away the excess portion of the fourth intermediate material 26 (or the portion that exists in the range of approximately from 10 percent to 25 percent from the surface of the fourth intermediate material 26), the outer ring 3 with the shape indicated by the chain lines in FIG. 8 (F) and FIG. 9 is obtained. Furthermore, the sections corresponding to the double-row outer ring raceways 2 formed in the inner peripheral surface of the outer ring 3, is subjected to processes such as a grinding and superfinishing to enhance the surface characteristics of the double-row outer ring raceways 2.
Incidentally, for the raw material 10 for making the outer ring 3, a column shaped material is used which is made by cutting to predetermined lengths a long piece of material with a circular cross-section that has been extrusion-molded by a steelmaker. The fact that the composition (cleanliness) of the column shaped raw material 10 obtained in this manner is not uniform, that is, in the range of within 40 percent from the center of the raw material 10 (or in the central column section of the raw material 10 which exists from the center to 40 percent in the radial direction), non-metallic inclusions are apt to be contained and thus the cleanliness is low, is already well known as disclosed in JP 2006-250317 (A) and the like. Also known is that, in the range of within 20 percent from the outer peripheral surface of the raw material 10 (or in the cylindrical section of the raw material 10 which exists on the outer peripheral surface side with respect to the range of from the center to 80 percent in the radial direction), the cleanliness is low due to the susceptibility to the presence of oxides and non-metallic inclusions. In other words, in the middle section in the radial direction of the raw material 10 (or in the range of from 40 percent to 80 percent from the center in the radial direction of the raw material 10), there is a metal material having a high degree of cleanliness in the circumferential direction, and there is a metal material having a low degree of cleanliness in the section closer to the center in the radial direction and in the section closer to the outer peripheral surface of the raw material 10. If the metal material with low cleanliness is exposed on a portion of the double-row outer ring raceways 2 provided on the inner peripheral surface of the outer ring 3, that makes rolling contact with the rolling surface of the ball 6 (FIG. 7), ensuring the rolling fatigue life of this portion is difficult.
Taking these circumstances into consideration, and also taking variations in the distribution of oxides and non-metallic inclusions within the material as well as various differences that occur at the time of the manufacturing operation (such as compressive force) into consideration, the metal material present in the range of within 50 percent from the center in the radial direction of the raw material 10 and in the range of within 30 percent from the outer peripheral surface in the radial direction of the raw material 10 is preferably not exposed on at least the portions of the outer ring raceways 2 which make rolling contact with the rolling surface of the ball 6. In other words, on at least the portions of the outer ring raceways 3 which make rolling contact with the rolling surface of the ball 6, preferably the metal material present in the middle cylindrical portion 27 of the raw material 10 (the crosshatched parts in FIG. 8 (A) to FIG. 8 (F), and FIG. 9), is exposed.
Incidentally, when a forging process is used to manufacture the outer ring 3 in which the double-row outer ring raceways 2 are provided at two locations in the axial direction on the inner peripheral surface on both sides of the section having a small inside diameter, exposing the metal material present in the middle cylindrical portion 27 on raceway surfaces of the double-row outer raceway 2 is difficult. For example, when the outer ring 3 shown by the chain line in FIG. 9 is produced by the method such as shown in FIG. 8 (A) to FIG. 8 (F), the center side metal material 28 which is present in the central columnar portion, that is a range of from the center to 50 percent in the radial direction of the raw material 10, the middle metal material 29 which is present in the middle cylindrical portion 27, that is a range of from 50 percent to 70 percent in the radial direction of the raw material 10, and the outer peripheral surface side metal material 30 which is present in the outer peripheral surface side cylindrical portion, that is a range of within 30 percent from the outer peripheral surface in the radial direction of the raw material 10, are distributed throughout the outer ring 3 as shown in FIG. 9. For this outer ring 3, as described above, the fourth intermediate material 26 as shown by the solid line in FIG. 9 is produced by a forging process, after which the fourth intermediate material 26 is shaved off to the state shown by the chain line in FIG. 9, by machining and grinding processes, and completed as the outer ring 3.
In FIG. 9 which shows the fourth intermediate material 26 and the outer ring 3, if the middle metal material 29 present in the middle cylindrical portion 27 shown by the crosshatching is exposed at least at the part of the double-row outer ring raceways 2 which makes rolling contact with the rolling surface of the ball 6, the rolling fatigue life of these double-row outer ring raceways 2 is ensured, which easily ensures the durability of the double-row angular ball bearing 1 which includes this outer ring 3. However, as is clear from FIG. 9, when the outer ring 3 is made by the conventional manufacturing method, the center side metal material 28 of the central columnar portion is exposed on the entire surface of one of the double-row outer ring raceways 2 (the lower outer ring raceway 2 in FIG. 9). For example, the arrows a in FIG. 9 indicate the direction of action of the load applied from the balls 6 to the outer ring raceways 2 (see FIG. 7) in the case where the contact angle of the balls 6 is 40 degrees (the complementary angle of the contact angle relative to the center axis is 50 degrees). If the middle metal material 29 is present at the part indicated by the arrows a on the chain line in FIG. 9 which indicates the cross-sectional shape of the outer ring 3, then the rolling fatigue life of the outer ring raceways 2 can be ensured easily. However in relation to the lower inner ring raceway 2 of FIG. 9, the center side metal material 28 is present in the part on the chain line indicated by the arrows a. Consequently, in conventionally known methods of manufacturing bearing outer rings, the degree of freedom in the design for ensuring the durability of the double-row angular ball bearing 1 is limited.